Rust By Example

As an introduction to this section, to borrow from the official docs,
"one should try to minimize the amount of unsafe code in a code base." With that
in mind, let's get started! Unsafe annotations in Rust are used to bypass
protections put in place by the compiler; specifically, there are four primary
things that unsafe is used for:

dereferencing raw pointers

calling functions or methods which are unsafe (including calling a function
over FFI, see a previous chapter of the book)

Raw pointers * and references &T function similarly, but references are
always safe because they are guaranteed to point to valid data due to the
borrow checker. Dereferencing a raw pointer can only be done through an unsafe
block.

Some functions can be declared as unsafe, meaning it is the programmer's
responsibility to ensure correctness instead of the compiler's. One example
of this is std::slice::from_raw_parts which will create a slice given a
pointer to the first element and a length.

For slice::from_raw_parts, one of the assumptions which must be upheld is
that the pointer passed in points to valid memory and that the memory pointed to
is of the correct type. If these invariants aren't upheld then the program's
behaviour is undefined and there is no knowing what will happen.